Direct Z-scheme heterojunction impregnated MoS2–NiO–CuO nanohybrid for efficient photocatalyst and dye-sensitized solar cell

In this present work, the preparation of ternary MoS2–NiO–CuO nanohybrid by a facile hydrothermal process for photocatalytic and photovoltaic performance is presented. The prepared nanomaterials were confirmed by physio-chemical characterization. The nanosphere morphology was confirmed by electron microscopy techniques for the MoS2–NiO–CuO nanohybrid. The MoS2–NiO–CuO nanohybrid demonstrated enhanced crystal violet (CV) dye photodegradation which increased from 50 to 95% at 80 min; The degradation of methyl orange (MO) dye increased from 56 to 93% at 100 min under UV–visible light irradiation. The trapping experiment was carried out using different solvents for active species and the Z-Scheme photocatalytic mechanism was discussed in detail. Additionally, a batch series of stability experiments were carried out to determine the photostability of materials, and the results suggest that the MoS2–NiO–CuO nanohybrid is more stable even after four continuous cycles of photocatalytic activity. The MoS2–NiO–CuO nanohybrid delivers photoconversion efficiency (4.92%) explored efficacy is 3.8 times higher than the bare MoS2 (1.27%). The overall results indicated that the MoS2–NiO–CuO nanohybrid nanostructure could be a potential candidate to be used to improve photocatalytic performance and DSSC solar cell applications as well.


Synthesis of MoS 2 -NiO
To begin with, 0.6 g of Na 2 MoO 4 •2H 2 O and 1.2 g of C 3 H 7 NO 2 S were dissolved in 40 mL of DI water, and 0.4 g of the NiO as prepared was dissolved in 40 mL of DI water separately and stirred for 1 h.The stirred solution was then mixed drop by drop, and the above mixture was sonicated for 15 min.The mixed solution was transferred into a 100 mL autoclave and maintained at 180 °C for one day.The sample was filtered washed and dried at 80 °C for half a day.Finally, the MoS 2 -NiO composite is obtained.

Synthesis of MoS 2 -NiO-CuO
In 40 mL of DI, 0.4 g of Na 2 MoO 4 •2H 2 O and 0.8 g of C 3 H 7 NO 2 S were dissolved, and subsequently, 0.25 g of the as-prepared NiO and 0.18 g of CuO were dissolved separately in 40 mL of DI water and stirred for 1 h.The stirred solution was then mixed drop by drop, and the above mixture was sonicated for 15 min.The mixed solution was transferred into a 100 mL autoclave and treated at 180 °C for one day.The sample was centrifuged, filtered, washed, and dried at 80 °C for half a day.Finally, the MoS 2 -NiO-CuO nanohybrid is obtained.

DSSC fabrication
The doctor-bladed method has been used to prepare the two active electrodes.The prepared samples were mixed with PVDF (0.95:0.05) and a few drops of NMP solution to form a colloid used as the counter electrodes.After that, the colloid was deposited on the FTO glass.To prepare photoanode, the commercial TiO 2 nanoparticles were mixed with Triton X-100, polyethylene oxide, and polyethylene glycol to create a TiO 2 slurry paste.The TiO 2 slurry was coated on the FTO plate and annealed at 500 ºC for 3 h and the photoanode was dipped with 3 mM N719 dye solution for one day.The gel electrolyte was prepared by using 0.6 M of 1-propyl-2,3-dimethylimidazolium iodide, 0.5 M of 4-tert-butylpyridine, 0.1 M of LiI, 0.05 M of I 2 , and 3% w/w of polyethylene oxide and 5 mL of acetonitrile with continuous stirring for half day 25 .Finally, the photoelectrode was accumulated with the as-prepared counter electrode convoyed with the injection to a few drops of electrolyte between the two electrodes and fabricated the complete DSSC device 7 .
The following formulas were used to determine the fill factor (FF%) and power conversion efficiency (PCE%),

Photocatalytic degradation
The photocatalytic experiment was conducted in a cylindrically shaped photocatalytic system under 500-W halogen light.To prepare the reaction solution, 0.02 g of sample and 20 ppm of methyl orange (MO) or crystal violet (CV) dye were mixed with 100 mL of DI water and this reaction solution was placed into the dark.The adsorption and desorption equilibrium was reached when the suspension was continuously stirred in a dark place for 1 h.After adsorption and desorption equilibrium processes, the dye solution was placed under 500-W halogen light.While UV-visible light irradiation on the dye solution, 3 mL of suspension is continuously withdrawn at 30 min intervals and the solution is centrifuged.The concentration of the recovered centrifuged dye solution was then measured using a UV-vis spectrometer.The degrading efficiency, first order kinetics and second order kinetics of the photocatalyst was calculated using the following equations.
(1) www.nature.com/scientificreports/Here, C and C 0 signified initial and final concentrations of the dye, k 1 is the pseudo-first-order rate constant (min −1 ) and k 2 is the pseudo-second-order rate constant (M −1 min −1 ), respectively.The stability test was carried out four times, each time maintaining the same settings as the previous photocatalytic experiment.

Scavengers test
The active species most strongly associated with the degradation process are also identified using a trap test.For the same experiment described above, the three kinds of 1 mmol scavengers for hydroxyl (OH), electrons (e − ), and superoxides ( • O 2 − ) radicals were taken: 2-propanol (IPA), silver nitrate (AgNO 3 ), and benzoquinone (BQ).After the first run was finished, the photocatalyst was removed from the dye suspension and repeatedly rinsed with DI water.The photocatalyst was then employed in the next cycle after being dried at 80 °C for 8 h.To determine the stability of the catalyst, four cycles of photocatalytic tests were conducted and the remaining catalyst was studied by XRD and SEM analysis.

Characterizations
The crystal phase purity of the obtained samples was examined by a Rigaku Miniflex X-ray diffractometer equipped with a Cu-Kα radiation source (λ = 1.5406Å).The FTIR spectrum of obtained samples was measured by Brucker Tensor 27 spectra.The Phi Versaprobe III type of X-ray photoelectron spectrometer was used to examine the elemental composition of the prepared samples.Ultraviolet and visible absorption diffuse reflectance spectra (UV-Vis-DRS) and photoluminescence (PL) studies were recorded by the Perkin Elmer Lambda 25 and FP-8200 fluorescence spectrometers.A Carl Zeiss (USA) model scanning electron microscope (SEM) attached to an EDAX detector and a JEOL JEM-2100 model transmission electron microscope (TEM) working at 200 kV were used to determine the morphology of the obtained samples.The thickness of the DSSC was measured using the SJ-301 Mitutoyo Surface Profilometer.The Scientech (SS 50 K, AAA)-EM employing an A.M. 1.5 G solar simulator with a Keithley 2400 m was utilized to investigate the characterizations of the fabricated DSSC.A PG-LYTE 1.0 electrochemical workstation was used for determining the photocurrent correspondence of the fabricated DSSC.

XRD analysis
The XRD patterns were used to investigate the crystal phase growth and purity of the obtained samples, as shown in Fig. 1.The crystal planes (002), (100), ( 110), ( 008) and ( 116

FTIR analysis
The FTIR spectra were recorded to confirm the presence of components inside the binary and ternary composites, and the findings are shown in Fig. 2a,c at 4000 to 400 cm −1 and the particular wave number between 1200 to 400 cm −1 are shown in Fig. 2b,d.The FTIR spectra reveal the unique bands produced by the elemental bonds along with the stretching and forming vibrations of Mo-S, Ni-O, and Cu-O in the samples.From the FTIR spectra of MoS 2, the band at 565 cm −1 produced from the data indicates the vibration of the Mo-S bond 29 .In contrast, the S-S bond produces a band at 916 cm −1 , while the Mo-O stretching vibrations produce bands at 1088 and 1167 cm −1 , respectively.The stretching modes of the Ni-O and Cu-O metal-oxygen bonds are associated with the peaks at 485 and 556 cm −1 , respectively 30 .Bands characteristic of MoS 2 , NiO, and CuO may be seen in the binary composites of MoS 2 -NiO, as well as in the MoS 2 -NiO-CuO nanohybrid.However, the MoS 2 peaks are more prominent than the NiO and CuO addition ratios.Assigned to the O-H group, C-H, C=C, and C-O-C stretching vibrations, have peaks that occur 31,32 at 3000-3750, 2333-1647, 1531-1306, and 3000-3750 cm −1 , respectively.According to the FTIR analysis, the ternary MoS 2 -NiO-CuO nanohybrid contains all of the peaks, indicating that the heterojunction nanohybrid was successfully constructed.

XPS analysis
The surface composition and oxidation states of the synthesized samples were further investigated by XPS investigation. ) and have been in agreement with the XPS data in the earlier report 33 .In the ternary hybrid, the Mo 3d spectra exhibited the characteristic peak at    The above results suggest that a Z-scheme MoS 2 -NiO-CuO nanohybrid heterojunction has been effectively constructed, and the close interaction between MoS 2 -NiO and CuO is favorable to electron transport, which is helpful for the catalytic process 16,38 .

Morphology analysis
The morphology of the synthesized MoS 2 -NiO-CuO nanohybrid was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations.The SEM images are shown in Fig. 4, where MoS 2 is built by a nonuniform nanoparticle (Fig. 4a,b) structure and NiO displays the nanoparticles structure (Fig. 4c,d).The MoS 2 -NiO has a surface morphology that reveals the formation of mixed nanoparticles structure (Fig. 4e,f).The single phase self-growth of CuO nanospheres and MoS 2 nanoparticles were decorated on the surface of NiO to develop MoS 2 -NiO-CuO nanohybrid heterostructure can be seen in Fig. 4g,h.Following the procedure of adding CuO, the nanosphere arrays have also been successfully retained.Notably, the surface of the ternary MoS 2 -NiO-CuO becomes fairly rough and transparent, in contrast to the smooth surfaces of MoS 2 and NiO nanoparticles. Figure 6 shows the TEM images of (a-c) MoS 2 -NiO and (d-f) MoS 2 -NiO-CuO hybrid.When the TEM image of MoS 2 -NiO and MoS 2 -NiO-CuO nanohybrid is compared with their SEM images, it is clear that the structure is still intact.The TEM images of MoS 2 -NiO exhibited NiO nanoparticles combined with MoS 2 .In MoS 2 -NiO-CuO nanohybrid, the CuO nanosphere are successfully combined with the MoS 2 and NiO nanoparticles, then to develop a MoS 2 -NiO-CuO nanohybrid heterostructure as shown in Fig. 6d-f.The observed lattice fringe spacing of 0.64 nm, 0.23 nm and 0.25 nm and these values correspond to the (002), ( 111) and (111) crystal planes of MoS 2 , NiO and CuO, as shown in Fig. 6d.After the addition of the CuO, it becomes quite clear to observe that the MoS 2 and the NiO nanoparticles have been completely transferred into the nanosphere formation.The presence of CuO in the interfacial layer that exists in between MoS 2 and NiO is further evidence that the MoS 2 -NiO-CuO nanosphere structure has been formed.Based on morphological analysis, the MoS 2 -NiO-CuO nanohybrid morphologies provide numerous active site interfaces between MoS 2 -NiO and CuO.So the MoS 2 -NiO-CuO nanohybrid can improve the photovoltaic and photocatalytic capabilities with excellent light response 40 .

UV and PL analysis
The optical absorption properties of the catalysts are studied by UV-Vis diffuse reflectance spectra.Figure 7a shows that MoS 2 , NiO, and CuO have apparent UV-visible-light absorption peaks at 416 and 662 nm for MoS 2 41 , 211 and 237 nm for NiO 42 and 214, 280, and 467 nm for CuO 31 .The enhanced absorbance of the binary composites at 266, 405, and 666 nm for MoS 2 -NiO, indicates that the addition of MoS 2 improves the efficiency with which NiO uses visible light.Furthermore, the obtained MoS 2 -NiO-CuO nanohybrid exhibited significantly increased visible-light absorption after the addition of CuO at the range between 210 to 720 nm, specifically at 384, 441, and 598, 659, and 719 nm 31 .Absorption of UV-visible light by the ternary material was significantly enhanced in comparison to the binary and ternary composites, with a conspicuous absorption peak observable between 200 and 680 nm in the spectrum.Based on the slope of the tangent line of the plot of (αhν) 2 vs. photon energy (hν), the band gap energies of MoS 2 , NiO, CuO, MoS 2 -NiO, and MoS 2 -CuO-NiO nanohybrid were determined to be 1.96, 2.28 43 , 2.16, 2.1, and 2.07 eV, respectively as shown in Fig. 7b.UV-DRS results reveal that the adsorption edge is expanded from the UV area to the visible range, leading to greater electron-hole pairs in the ternary composites than in the pure and binary composites.The reduced band gap indicates that the ternary composite increases the conductivity and aids in improved photocatalytic and photovoltaic performance of the MoS 2 -NiO-CuO nanohybrid 44 .Effective charge transfer and separation of the as-prepared heterojunctions may be assessed by measuring the PL spectrum, and the emission produced by the recombination rate of electron-hole pairs.The photocatalytic activity of a given photocatalyst is strongly correlated with the intensity of its   sinusoidal perturbation to the system and analyzing the resulting current response.The electrochemical behavior and charge transfer processes occurring within the cell.One characteristic feature commonly observed in the EIS spectrum of DSSCs is the presence of two distinctive semicircles, each corresponding to specific electrochemical interfaces within the cell.It was analyzed from 10 mHz to 10 kHz with bias of open circuit voltage, Fig. 8d, the Nyquist plot that was obtained from the EIS spectra is displayed.The high-frequency semicircle typically appears in the EIS spectrum and is associated with the charge transfer resistance (R ct-1 ) at the electrolyte/counter electrode interface.This region represents the impedance encountered during the transfer of electrons between the counter electrode and the electrolyte.A larger semicircle suggests higher charge transfer resistance, potentially indicating limitations in the kinetics of the electrochemical reactions at this interface.The low-frequency semicircle is associated with the interaction between the photoanode and the electrolyte.This region, denoted as R ct-2 , signifies the charge transfer resistance at the photoanode/electrolyte interface.A detailed analysis of R ct-2 aids in understanding the efficiency of charge transfer reactions in this critical region.

Photocatalytic activity analysis
A provided UV-visible light source (500-W halogen) is critical in converting degradation products into photosynthetic reactions.There is no change in dye concentration in the absence of a catalyst.However, when a catalyst is used in a photocatalytic reaction, the concentration of dye varies with the capacity of the catalyst produced.The UV-Vis absorption spectra of photodegraded dyes for prepared samples are shown in Fig S .(2)and S.(3), indicating the photocatalytic performance of the dyes produced by the photocatalyst by (1) CV and (2) MO dye degradation at 80 and 100 min, respectively.The UV-Vis absorption spectra presented in Fig S .(2)and S.(3) show maximum peak positions at 586 nm and 465 nm corresponding to the CV and MO dye, respectively.The absorbance intensity of CV and MO dye with photocatalyst decreases with increasing time of UV-Vis light irradiation.
The self-degradation process of CV dye is studied without using any catalysts, and it is found that there is no significant change in CV dye concentrations.Figure 9a   Table 1.Photovoltaic parameters of the as fabricated DSSC.
Prepared materials V oc (V) J sc (mA cm −2 ) Fill factor (%) •OH radicals and e − radicals.The • O 2 − plays a critical role in the photocatalytic degradation of CV and MO by MoS 2 -NiO-CuO nanohybrid.
The two most crucial elements for the practical use of a catalyst are its reusability and stability.To study the photocatalytic effectiveness of the MoS 2 -NiO-CuO nanohybrid for the degradation of CV and MO, the process was repeated four times.As can be seen in Fig. 11, MoS 2 -NiO-CuO nanohybrid degradation efficiency decreases only slightly, with 90% and 89% of (c) CV and (d) MO degraded, after four cycles of recycling respectively.In addition, after the fourth recycling process, the catalyst was washed, cleaned, and dried in a hot air oven to determine its stability.The stability of the catalyst was analyzed by SEM images as shown in Fig. 12.Before and after the photocatalytic processes, the MoS 2 -NiO-CuO nanohybrid did not undergo any noticeable changes in phase, morphology, and elemental composition as illustrated in Fig. 13 [59][60][61][62][63][64][65] and methyl orange (MO) dye [66][67][68][69][70][71][72] by presented work and previous works as shown in Tables 3 and 4. From the table of overall photocatalytic analysis results, the freshly prepared MoS 2 -NiO-CuO ternary hybrid exhibits the most effective degradation performance for CV and MO dyes.

Charge transfer mechanism
In contrast, the mechanism of the traditional charge transfer process is different from the Z-scheme heterojunction.A Z-scheme type charge transfer process has been presented in the presence of MoS 2 -NiO-CuO nanohybrid to generate the effective charge separation between the CB of MoS 2 and the VB of NiO and CuO, which may serve as a feasible approach to increase and extend charge life.Due to the proximity of these two types of semiconductors, P-N junctions appeared on the surface of the p-type semiconductor of MoS 2 and NiO on the n-type semiconductor 73 .To understand the electron transfer mechanisms of the produced materials, it is necessary to evaluate the reduction and oxidation capacities of electrons and holes within the MoS 2 -NiO-CuO hybrid.The Eqs. ( 6) and (7) given below can be used to calculate the energy level of the conduction band (E CB ) and valence band (E VB ) in the MoS 2 -NiO-CuO hybrid.8)-( 14):    www.nature.com/scientificreports/ The better DSSC and photocatalytic performances of the MoS 2 -NiO-CuO nanohybrid may be attributed to the following influences.(i) Suitable surface morphologies could be modified by the incorporation of metal oxide, and there is no obvious aggregation of the full sphere, which indicates that the substance has a high degree of stability 78 .(ii) The electron transfer rate of the electrode may be increased by the interaction of different components, which in turn encourages the reduction process of I 3 − and the collection of photogenerated electrons in an external circuit.All of these have the potential to prevent photo-generated electrons from recombining, thereby decreasing dark current production and raising J sc in DSSC 79,80 .(iii) Ni and Cu are first-row transition metal ions and they can couple with the sulfur edges of molybdenum to form the strongest chemical bonding, which accelerates efficient proton adsorption and promotes charge transfer between the CuO, NiO, and MOS 2 .In previous reports, the active sulfur atoms on the exposed edges of MoS 2 increased redox activity and these unsaturated active sulfur atoms could efficiently bind with H + in the solution, which can easily improve the redox activities in the reaction [81][82][83] .(iv) The significant dual performances might be attributed to the carrier's recombination activities being inhibited.The band gaps of MoS 2 -NiO-CuO nanohybrids are smaller, there are more recombination centers in the Z-scheme heterojunction, and there are more ways for electrons and holes to separate.These factors help separate charges better and absorb more visible light, as well as significantly increasing the photocatalytic and DSSC solar cell performances 84 .

Conclusion
In summary, we have prepared MoS 2 -NiO-CuO nanohybrid as Z-scheme heterojunction were synthesized by hydrothermal method.The XRD, FTIR, XPS, and FTIR analyses confirmed the construction of the MoS 2 -NiO-CuO nanohybrid.The MoS 2 -NiO-CuO hybrid morphological studies of SEM and TEM images of both showed the same nanospheres shape, and all their elemental components appear in the EDAX spectrum.The MoS 2 -NiO-CuO nanohybrid explored photocatalytic activity are 95 and 93% for CV and MO dye degradation under UV-Vis light and the PCE (%) of as fabricated MoS 2 -NiO-CuO DSSC solar cell is 3.8 times higher than the MoS 2 .The synergistic impact of CuO introducing and NiO nano-grafting to collectively influence MoS 2 intrinsic intercalation structure was discovered to increase the photocatalytic and photovoltaic activity due to the quick recombination of the photo-induced electron-hole pairs.The ongoing development work of Z-schemebased MoS 2 -NiO-CuO nanohybrid will improve future applications in the areas of organic pollutant removal and emergent energy storage in a low-cost and eco-friendly manner.
Fig. S.1 (a).In the O 1s spectra of MoS 2 , three peaks for the O 1s are observed at 530.57, 531.54, and 532.65 eV, whose characteristic peaks correspond to binding energies in MoO, O 1s, and MoO 3 .The O 1s spectra of the MoS 2 -NiO-CuO nanohybrid are shown in Fig S.1.(a), four peaks for the O 1s are observed at 530.98, 531.81, 532.9, and 533.63 eV, which corresponds to the binding energies of MoO, O 1s, NiO, and CuO 37,38 .Finally, Fig. S.1.(b) shows the C 1s XPS spectra of MoS 2 and MoS 2 -NiO-CuO nanohybrid, the intensive peak at 283.74 and 285.07 eV corresponds to C=C and C-O, respectively 39 .The binding energies of Mo 3d, S 2p, and O 1s were compared with pure MoS 2 and MoS 2 -NiO-CuO nanohybrid, showing that MoS 2 -NiO-CuO nanohybrid has higher binding energies at 0.1, 0.4 and 0.27 eV.The XPS results strongly confirmed that there were indeed interactions between CuO and MoS 2 -NiO when CuO was loaded onto MoS 2 -NiO to form the MoS 2 -NiO-CuO nanohybrid.

Figure 5
shows the EDAX spectra of (a) MoS 2 , (b) NiO, (c) MoS 2 -NiO, and (d) MoS 2 -NiO-CuO nanohybrid.The EDAX spectra of pure MoS 2 the presence of the elements Mo, S, and O which consist of 51, 28, and 21 by weight %.The weight percentages of the Mo, S, Ni, Cu, and O components in the synthesized MoS 2 -NiO-CuO nanohybrid are 38, 31, 10, 7, and 14%, respectively (Fig.5d).The Fig.5edisplays the element mapping of the as-prepared samples MoS 2 -NiO-CuO nanohybrid.As a result of the discovery of impurities in the synthetic MoS 2 -NiO-CuO nanohybrid, it has been determined that there are no extra elements for the other components, indicating that the synthesized nanohybrid is high purity.
photoluminescence.MoS 2 displays a large and wide emission peak at about 408 nm45 with a matching excitation wavelength of 360 nm, corresponding to the rapid recombination rate of photogenerated carriers, as illustrated in Fig.7c.When MoS 2 -NiO and MoS 2 -NiO-CuO nanohybrid are compared to pure, the PL intensity gradually decreases.Furthermore, the PL intensities for the MoS 2 -NiO-CuO nanohybrid are gradually reduced due to the mixture of CuO, which is indicative that the addition of CuO is aiding in improved separation efficiency and inhibiting recombination of charge carriers at the heterojunctions45,46 .
(a) XRD and (b-d) EDAX.From the stability test, no obvious variations in the photocatalytic activity of MoS 2 -NiO-CuO nanohybrid were observed after the decomposition of CV and MO dyes, indicating excellent stability and recyclability of MoS 2 -NiO-CuO nanohybrid.The photocatalytic degradation comparison of crystal violet (CV)
www.nature.com/scientificreports/ in MoS 2 than NiO.After the formation of MoS 2 -NiO-CuO ternary nanohybrid, the peak intensity of MoS 2 in MoS 2 -NiO-CuO nanohybrid was enhanced, indicating the influences of CuO in MoS 2 -NiO.The discussion of the elemental mapping stated before is supported by this information.

Table 2 .
Comparative behavior of DSSCs with different hybrid counter electrodes.

Table 3 .
Comparison photocatalytic degradation of crystal violet (CV) dye by presented work and previous works. S.

Table 4 .
Comparison photocatalytic degradation of methyl orange (MO) dye by presented work and previous works.